1. Field of the Invention
The present invention relates to a process and apparatus for the separation of gaseous mixture containing carbon dioxide as main component. It relates in particular to processes and apparatus for purifying carbon dioxide, for example coming from combustion of a carbon containing fuel, such as takes place in an oxycombustion fossil fuel or biomass power plant.
2. Related Art
Various techniques based on solvent, sorbents, and membranes have been proposed for CO2 capture from power plants or industrial sources. Some techniques utilize a medium (e.g., amines) for capturing CO2 through chemical affinity. However, the energy needed for regenerating the medium (having chemical affinity for CO2) is significantly high. Other solvents and adsorbents capture CO2 through physical affinity. While the energy necessary for regenerating such solvents and adsorbents is relatively lower than that the media having chemical affinity for CO2, they typically have a relatively lower capacity for CO2. On the other hand, membranes use a combination of physical affinity and diffusivity. The driving force for transport through membranes is the difference between CO2 partial pressure across the membrane (i.e., the feed partial pressure minus the permeate partial pressure).
Regardless of the technique employed to recover CO2, high CO2 recoveries from feed gases is desirable for a variety of reasons. For example, the U.S. Department of Energy (DOE) has set a target recovery for recovering CO2 from power plants. As another example, high CO2 recoveries allow more CO2 product gas to be sold or used in order to recover the costs associated with the pre-treatment of the flue gas necessary for recovery. However, in the case of CO2 recovery utilizing membrane separation, as more and more CO2 is sought to be recovered, the driving force across the membrane decreases and approaches a pinch point beyond which additional recovery comes at the expense of high compression energy costs or high membrane surface areas. Thus, for some levels of CO2 recovery, this problem has the potential to increase capital and operating expenses to unsatisfactory levels.
Each of U.S. Pat. No. 8,617,292, U.S. Pat. No. 8,663,364, and U.S. Pat. No. 8,734,569 discloses that operation of membranes at relatively cold temperatures is highly effective for CO2 capture. Cold temperature operation leads to high membrane selectivity with negligible membrane permeance loss or even possibly an enhancement in membrane permeance. While operation of cold membranes is quite efficient, higher and higher CO2 recoveries may be desired without concomitant unsatisfactorily high increases in capital and operating expenses.
Membranes are known to be efficient for bulk separation of gases when the driving force is high. They have been used in combination with other, subsequent, gas separation techniques in order to achieve an overall CO2 recovery. Such hybrid systems are known where a membrane performs a bulk CO2 separation from natural gas followed by amine treatment of the lower concentration membrane residue stream. Hybrid combinations of solvent (e.g. piperazine) and membrane have also been studied for CO2 capture from flue gas.
One particular two unit separation process is disclosed by U.S. Pat. No. 8,728,201 including a membrane utilizing a vacuum on a permeate side that is followed with an absorption (solvent) to remove CO2 from the membrane residue. There is little integration between the two unit operations.
One particular U.S. Department of Energy funded project uses a costly and cumbersome plate and frame membrane system to operate with an air sweep at low pressures. In this approach, the membrane is placed in series—after the solvent unit or in parallel with the solvent unit (Freeman, et al. “Bench-Scale Development of a Hybrid Membrane-Absorption CO2 Capture Process (DE-FE0013118)”, Dec. 20, 2013 Kickoff Meeting).
Hybrid processes combining adsorption and membranes are also known. For example, U.S. Pat. No. 8,591,769 and U.S. Pat. No. 6,183,628 discuss membrane treatment of PSA vent gas to recover H2. However, if this technique was applied to flue gas, such a scheme would require use of a less optimum adsorbent that is exposed to many impurities.
WO14009449 A1 proposes to combine membrane and adsorption processes for moisture removal.
Membranes can be swept with a sweep gas in order to overcome the above-described membrane driving force pinch problem. U.S. Pat. No. 8,734,569 discloses that this can be done by diverting a small fraction of gas (that is derived from the low CO2 concentration residue) to sweep the permeate side of a membrane module. For a low sweep rate, the permeate CO2 concentration decreases marginally but the membrane area can be decreased significantly. However for high sweep rates, permeate CO2 concentrations can decrease significantly.
Another sweep concept, particularly applicable to CO2 capture from flue gas, utilizes a two step membrane process (Merkel, et al., “Power plant post-combustion carbon dioxide capture: An opportunity for membranes”, Journal of Membrane Science 359 (2010) 126-139). The 1st permeate at relatively high CO2 purity is sent for further CO2 purification. The 2nd membrane is swept with an air stream to achieve high CO2 recovery. The air stream is then sent to the boiler island where the recovered CO2 dilutes the overall stream, imposing a small energy penalty for combustion.